The present invention relates to magnetic resonance imaging (MRI) systems; and more particularly to devices which couple a radio frequency antenna to transmitter and/or receiver circuits in the system.
Magnetic resonance has been developed as an imaging method useful in diagnostic medicine. In magnetic resonance imaging (MRI), a body being imaged, such as a medical patient, is held within a uniform magnetic field oriented along a Z axis of a Cartesian coordinate system. Magnetic gradient fields G.sub.x, G.sub.y and G.sub.z are applied along the X, Y and Z axes to impress position information onto the MRI signals through phase and frequency encoding.
The net magnetizations of the nuclei in the body then are excited to precession by a radio frequency (RF) pulse that is applied through a switch and a coupler to an antenna located adjacent the object being imaged. The coupler splits the RF pulse into two signals I and Q in quadrature which are coupled to the antenna. In some instances the RF pulse is used to excite nuclei throughout most of the body, in which case the antenna is relatively large and can consume 2.5 KW of power, for example. In other instances a smaller antenna is used to excite nuclei in only a portion of the body, the head of a patient for example. In this latter case the small antenna is placed locally about that portion and requires less power.
The decaying precession of the spinning nuclei produces the MRI signal, which has an amplitude that is dependent, among other factors, on the number of precession nuclei per volume within the image body, termed the "spin density." The MRI signal induces two quadrature signals in the antenna. In the receiving mode, the signals from the antenna are switched from the coupler to a receiver.
The excitation and signal reception modes are repeated to acquire a number of "views", a view being defined as one or more MRI signal acquisitions made under the same X and Y gradient fields. The views then are processed to reconstruct an image of the object.
Conventional couplers for the RF antenna were quadrature hybrid devices comprising four one-quarter wavelength transmission lines connected as sides of a square network. In the excitation mode, the RF signal was applied to one corner of the square network and the outputs for the I and Q signals were at the next two adjacent corners, respectively. Each input and output of the network had the same impedance (e.g. 50 ohms). An equivalent "dummy load" impedance was connected to the fourth corner of the network. Two MRI systems employ an RF excitation signal having center frequencies of 21.3 MHz. or 63.86 MHz., depending upon the strength of the magnetic field. For a 21.3 MHz. excitation signal, each transmission line had to be approximately ninety inches (228 cm) long, whereas a thirty inch (76 cm) transmission line is required at 63.86 MHz. For these transmission line lengths, the enclosure for the coupler becomes extremely large. In the reception mode, the receiver was coupled to the fourth corner of the network in place of the dummy load and another dummy load was connected to the one corner instead of the excitation signal.
A quadrature hybrid network using lumped elements can be substituted for the transmission line network described above. One form of such a hybrid network comprises a set of capacitors connected in a square with each corner being connected to ground potential by an inductor formed by a relatively short transmission line. The signals are applied to and fed from the corners of the network. However, in this type of network the values of the inductive reactance and capacitance must be very precise or variable to properly balance the network. An imbalanced network will not produce excitation output signals that are exactly ninety degrees out of phase or of unequal amplitude. Unequal or out of quadrature signals adversely affect the nuclei excitation and the subsequent acquisition of MRI signals. An imbalanced network conversely affects the reception of signals from the antenna. Although variable capacitors theoretically could be used, thus making the coupling network tunable, the power levels dictate very large and bulky capacitors and such devices are prone to arcing.